In the field of optical fiber communication, wavelength division multiplexing transmission systems are remarked as being available for a drastic increase in transmission capacity. In optical amplifying repeating transmission systems configured to amplify and repeat optical signals to transmit them over a long distance, optical band equalizing technologies are indispensable to flatten wavelength amplifying characteristics of optical amplifying repeater. Wavelength amplifying characteristics vary with the state of pumping of the optical amplifier. As a result, flattened amplifying characteristics may vary and may become non-flat relative to the wavelength due to a trouble in the pumping light source, for example. It is therefore important for maintenance and operation of an optical transmission system to monitor the output of the optical amplifier to know power of each signal wavelength component in the wavelength division multiplexed light.
An optical spectrum analyzer is usually used as a device for measuring optical power of each signal wavelength component in a wavelength division multiplexed optical signal. However, in optical repeaters which are used in locations very difficult to install them, such as in optical submarine cables, and are required to be miniaturized and highly reliable, it is almost impossible to use conventional optical spectrum analyzers.
To overcome the problem, a structure has been proposed that detects optical power by superposing a tone signal of a low frequency (approximately several 10 kHz, herein called the measurement frequency because it is the direct subject of measurement) onto each signal wavelength component in a wavelength division multiplexed optical signal, transmitting the superposed optical signals into an optical fiber line, and extracting the tone frequency component from the signals in each optical repeater. FIG. 13 is a block diagram of a general construction of the proposal.
Terminal stations 110, 112 respectively have optical sending equipment 110S, 112S, and optical receiving equipment 110R, 112R. The terminal stations 110, 112 are connected to each other by an optical repeating transmission line 114 including a pair of optical fiber transmission lines 114a, 114b. An optical signal output from the optical sending equipment 110S in the terminal station 110 travels through the optical fiber transmission line 114a and enters into the optical receiving equipment 112R in the terminal station 112. An optical signal output from the optical sending equipment 112S in the terminal station 112 travels through the optical fiber transmission line 114b and enters into the optical receiving equipment 110R in the terminal station 110. Typically, a plurality of optical repeaters 116 are provided along the optical repeating transmission line 114. FIG. 13, however, illustrates only one optical repeater 116 for simplicity.
The optical sending equipment 110S includes optical transmission signal senders 118-1 through 118-n which convert digital data #1 through #n into optical signals with different frequencies .lambda.1 through .lambda.n, respectively and output them. The optical signals with frequencies .lambda.1 through kn output from the optical transmission signal senders 118-1 through 118-n are wavelength-multiplexed by a wavelength multiplexer 120, and sent out onto the optical fiber transmission line 114a.
In order to measure the optical power of each wavelength light, the optical sending equipment 110S further includes an oscillator 122 which oscillates at a predetermined low frequency fs (measurement frequency), and a tone signal of the measurement frequency fs output from the oscillator 122 is applied to the optical transmission signal senders 118-1 through 118-n via switches 124-1 through 124-n. When one of the switches 124-1 through 124-n is closed, the tone signal output from the oscillator 122 is delivered to corresponding one of the optical transmission signal senders 118-1 through 118-n. One of the optical transmission signal senders 118-1 to 118-n supplied with the tone signal output from the oscillator 122 slightly modulates the optical transmission signal in intensity with the tone signal (frequency fs) output from the oscillator 122. For example, the injection current of the light source of each optical transmission signal sender 118-1 to 118-n is slightly amplitude-modulated with the tone signal output from the oscillator 122. The rate of the amplitude modulation is usually within 5% not to affect the transmission characteristics of the optical transmission signal. FIG. 14 shows a time waveform of an optical transmission signal intensity-modulated with a sinusoidal wave output from the oscillator 122.
Also the optical sending equipment 112S in the terminal station 112 have the same construction and operates in the same manner as the optical sending equipment 110S in the terminal station 110.
In the optical repeater 116, optical amplifiers 130a, 130b optically amplify optical signals (wavelength division multiplexed optical signals with wavelengths .lambda.1 through .lambda.n) introduced from the optical fibers transmission lines 114a, 114b. Dividers 132a, 132b respectively output most of the optical outputs of the optical amplifiers 130a, 130b onto the optical fiber transmission lines 114a, 114b toward the terminal stations 112, 110 (or a subsequent optical repeater) and apply small amounts of them to measuring circuits 134a, 134b. The measuring circuits 134a, 134b, which will be explained later in greater detail, extract optical intensity components (that is, optical output component of an optical transmission signal sender 118-i (i=1.about.n) corresponding to a closed switch 124-i) variable with oscillation frequency fs of the oscillator 122 in the optical sending equipment 110S, and measure their optical intensity. Modulators 136a, 136b slightly modulate amplification gains of the optical amplifiers 130b, 130a in accordance with the digital data (or their coded data) resulting from measurement by the measuring circuits 134a, 134b in order to deliver the results of measurement by the measuring circuits 134a, 134b.
In this manner, the optical signal from the terminal station 112 toward the terminal station 110 results in being intensity-modulated in response to the measured optical power of light with a predetermined wavelength in the wavelength division multiplexed light output from the terminal station 110, and the intensity-modulated optical signal is sent out onto the optical fiber transmission line 114b via the divider 132b and enters into the optical receiving equipment 110R in the terminal station 110. Similarly, the optical signal from the terminal station 110 toward the terminal station 112 is intensity-modulated in response to the measured optical power of light with a predetermined wavelength in the wavelength division multiplexed light output from the terminal station 112, and the intensity-modulated optical signal is sent out onto the optical fiber transmission line 114a via the divider 132a and enters into the optical receiving equipment 112R in the terminal station 112.
In the optical receiving equipment 112R in the terminal station 112, the optical signal introduced from the optical fiber transmission line 114a is divided by a divider 140 into a part to be processed as a received signal light and a part to be processes as power measurement data. The divisional light to be processed as the received signal light is applied to a wavelength demultiplexer 142. The wavelength demultiplexer 142 demultiplexes the light from the divider 140 into different wavelengths .lambda.1 through .lambda.n, and applies them to optical transmission signal receivers 144-1 through 144-n. The optical transmission signal receivers 144-1 through 144-n reproduce data #1 through #n from optical signals with wavelengths .lambda.1 through .lambda.n from the wavelength demultiplexer 142, and output them. Thus, the data #1 through #n are transmitted from the terminal station 110 to the terminal station 112. Also in the optical receiving equipment 110R in the terminal station 110, the same processing is executed.
The light divided by the divider 140 to be processed as the power measurement data is introduced into a photodetector 146 and converted into an electric signal. A data reproducing circuit 148 extracts and reproduces power measurement data from the electric signal output from the photodetector 146. A display/record device 150 displays the power measurement data information from the data reproducing circuit 148 on a monitor screen and/or records it on a recording medium. An administrator in the terminal station 112 for managing the optical transmission system looks at the power measurement data information, and adjusts the optical output power of each transmission signal sender in the optical sending equipment 112S.
The same processing as that of the optical receiving equipment 112R in the terminal station 112 is executed also in the optical receiving equipment 110R in the terminal station 110. An administrator in the terminal station 110 for managing the optical transmission system looks at the optical power information of each wavelength output from a display/record device in the optical receiving equipment 110R, similar to the display/record device 150, and adjusts the optical output power of each optical transmission signal sender 118-1 through 118-n in the optical sending equipment 110S.
FIG. 15 is a block diagram generally showing the construction of the measuring circuit 134a. The measuring circuit 134b also has the same construction as the measuring circuit 134a. A photodetector 152 converts light (wavelength division multiplexed light with wavelengths .lambda.1 to .lambda.n) from the divider 132a into an electric signal, and amplified and outputs it. The photodetector 152 may be of a low-speed type sufficient for extracting the oscillation frequency fs of the oscillator 122. In the case where NRZ codes are used as the coding system of the wavelength division multiplexed optical signal, and all coding rates are equal, a strong power spectral component accumulating optical signal spectral distributions of respective wavelengths as shown in FIG. 16 appears at the output of the photodetector 152. The output of the photodetector 152 is applied to a band pass filter 154 whose central frequency in the pass band is the tone signal frequency fs, and the frequency fs component is extracted from the output of the photodetector 152. The spectral distribution of an output from the band pass filter 154 is shown in FIG. 17. Here is shown that the fs frequency component slightly projects from the spectral component of the transmission signal within the pass band of the band pass filter 154.
A detector 156 detects the envelope of the frequency fs component from the output of the band pass filter 154. It is known that an optical power Ps and the level S of a tone signal with the depth of modulation factor m have the following relation. EQU S=a(mRPs).sup.2 (1)
where a is a coefficient indicating the loss from the optical amplifier 130a to the measuring circuit 134a and the gain of the photodetector 152, and R is the sensitivity to light of the photodetector 152. The coefficient a is calculated upon fabrication of the repeater 116 from the power Ps of the optical signal output from the divider 132a, after superposition of the tone signal, and the actual value of the tone signal level S measured by the measuring circuit 134a. Since the coefficient a, modulation factor m and light sensitivity R are already known, the optical power Ps can be known by measuring the tone signal level S. Since this conversion is typically made in the terminal stations 110, 112, output of the detector 156 is applied to the modulator 136a as an output of the measuring circuit 134a.
In this manner, by applying the tone signal output from the oscillator 122 to the optical transmission signal senders 118-1 through 118-n in sequence, the optical power of the signal light with respective wavelengths can be measured in the actually transmitted mode of the wavelength division multiplexed optical signal.
However, in conventional art, as the number of wavelengths to be wavelength-division-multiplexed increases, the power spectral density of the transmission signal increases at the output of the photodetector 152, and it causes deterioration of the S/N ratio at the output of the band pass filter 154. It results in serious degradation of the power measurement accuracy, or finally disables detection of the tone component, which means that the measurement of the optical power of each wavelength signal becomes impossible.
Although the S/N ratio at the output of the band pass filter 154 can be improved by increasing the modulation factor of tone, excessively deep modulation is undesirable because it invites deterioration of the transmission characteristics of the optical transmission signal.
The S/N ratio can be improved also by narrowing the pass band of the band pass filter 154. However, in the present state of art, in addition to the fact that the selection degree Q is up to about 1000, the use of too narrow pass band not only requires more strict control of the oscillation frequency fs of the oscillator 122 but also needs the use of a band pass filter 154 with a highly stable center frequency. These are the factors increasing the cost.